1. Field of the Invention
The present invention relates to a shift control apparatus for a transmission and more particularly to the shift control apparatus in which, when a vehicle travels on grades, a kick-down can be established at the same timing as traveling on an even road.
2. Discussion of Related Arts
In automatic transmissions, there are a continuously variable transmission (CVT) in which speed reduction ratios are changed continuously and a gear type automatic transmission in which gear ratios are changed through gear trains. In the continuously variable transmission, there are a belt drive type in which speed reduction ratios are changed by two variable groove width pulleys and a belt or chain winding around the pulleys and a troydal type in which speed ratios are changed by two discs and two power rollers.
On the other hand, in the gear type automatic transmission, there are a planetary type in which a plurality of gear trains composed of planetary gears are changed on one shaft and a parallel shafts type in which a plurality of gear trains provided between two or more shafts are changed by changeover mechanisms.
As disclosed in Japanese Patent Application Laid-open No. Toku-Kai-Hei 11-182665, thus automatically controlled speed ratios or gear ratios are obtained by referring to a shift schedule chart parameterizing vehicle speeds, throttle opening angles and the like and stored in a memory of a control unit. Thus, in case of a continuously variable transmission, optimum speed ratios are selected based on vehicle speeds, throttle opening angles and the like and stepless (continuous) shiftings are accomplished from a low speed up to an overdrive. Further, in case of a gear type automatic transmission, optimum gear positions are selected based on vehicle speeds, throttle opening angles and the like and a stepwise shifting is performed also from a low speed up to an overdrive.
This shift schedule chart is mostly established based on experiments and the like so as to be optimum under general traveling conditions. Hence, in case where a sharp acceleration is needed when passing, transmissions having a kick-down mode in which speed ratios or gear ratios rapidly increase or the gear position is shifted down so as to enable a sharp acceleration have been developed.
Further, some transmission can automatically establish a kick-down mode automatically based on an operating amount of an accelerator pedal determined by vehicle speeds.
However, in case where a vehicle enters a traveling on grades from a traveling on flat grounds, an accelerator pedal must be operated according to a gradient of road in order to maintain vehicle speeds.
Accordingly, when the operating amount of the accelerator pedal varies according to road gradients, some transmission having a kick-down mode operative based on the operating amount of the accelerator pedal enters the kick-down mode irrespective of an intention of a driver and as a result the driver feels uncomfortable.